Hemodialysis apparatus of the above described type generally have inlet and outlet conduits into the blood chamber of the dialyzer to form a blood flow path through the blood chamber. On the other hand, a dialysis fluid flow path is formed, starting at the dialysis fluid source and ending at a dialysis fluid outlet or drain. Separate dialysis fluid conduits connect, in series, the dialysis fluid source, the fresh dialysis fluid side or first partial chamber of the balance chamber, the dialysis fluid chamber of the dialyzer, the used dialysis fluid side or second partial chamber of the balance chamber, and finally the used dialysis fluid outlet. Furthermore, an ultrafiltration unit is connected to the fluid flow conduit between the dialysis fluid chamber of the dialyzer and the used dialysis fluid side or second partial chamber of the balance chamber.
A first group of valves or other shut-off devices includes a first valve arranged between the dialysis fluid source and the fresh fluid side of the balance chamber and a fourth valve arranged between the used fluid side of the balance chamber and the used fluid outlet. A second group of valves or shut-off devices includes a second valve arranged between the fresh fluid side of the balance chamber and the dialysis fluid chamber of the dialyzer and a third valve arranged between the dialysis fluid chamber and the used fluid side of the balance chamber. A dialysis fluid supply pump is arranged in the conduit between the dialysis fluid source and the fresh fluid side of the balance chamber. A second dialysis fluid pump is arranged in the conduit between the dialysis fluid chamber of the dialyzer and the used fluid side of the balance chamber.
A control and regulation unit controls the pumps and valves so that in a first operating cycle the dialysis fluid supply pump is activated and the first group of valves, including the first and fourth valves, is activated or opened, while the second group of valves, including the second and third valves, is deactivated or closed. In a second operating cycle, the second dialysis fluid pump is activated and the second group of valves, including the second and third valves, is activated or opened, while the valves of the first group are correspondingly deactivated or closed.
Hemodialysis apparatus using volumetric ultrafiltration control, typically operate with a closed dialysis fluid system, from which predetermined amounts of the dialysis fluid are removed or withdrawn. Because the system is closed, the withdrawn volume of fluid is made up or replaced by a corresponding volume of fluid which diffuses through the dialyzer membrane from the extracorporeal blood flow circuit and thus from the patient.
In order to achieve a substantially steady and constant flow of dialysis fluid from the dialysis fluid source on the one hand, to the dialyzer on the other hand, despite the closed arrangement of the fluid system, numerous hemodialysis apparatus include a balancing device that comprises two balance chambers. Each of the two balance chambers is divided into two partial chambers by a displaceable membrane. Thus, a total of four balance chamber portions or partial chambers is provided. Each of these four partial chambers is connected to an inflow conduit and an outflow conduit, resulting in a total of eight conduit connections to the four partial chambers of the two balance chambers. A respective shut-off device, such as a valve, is arranged in each one of the conduits, resulting in a total of eight separate shut-off valves.
The balance chambers are interconnected by conduits in such a manner, and the shut-off valves are appropriately controlled, so that one of the balance chambers is in a respective filling and emptying operating cycle while the other balance chamber is in the dialysis operating cycle. That is to say, in one balance chamber a first partial chamber is being filled with fresh dialysis fluid, whereby the displaceable membrane is displaced to make this first partial chamber larger, while tending to make the second partial chamber smaller, so that the used dialysis fluid is emptied from the second partial chamber. While the one balance chamber is in this filling and emptying operating cycle, the other balance chamber is in the dialysis operating cycle, whereby fresh fluid is pumped from the fresh fluid side of the balance chamber through the dialyzer and returned to the used fluid side of the balance chamber. As soon as the fresh dialysis fluid of the second balance chamber has been completely delivered through the dialysis fluid chamber of the dialyzer, then a cyclically alternating, controlled switching of the two balance chambers is carried out. As a result, the balance chamber of which the fresh fluid side has been filled now operates in a dialysis cycle, while the other balance chamber of which the fresh fluid side has been emptied is again refilled. In this manner, at each moment, one of the balance chambers is being filled with fresh dialysis fluid while the fresh fluid that has just been filled into the other balance chamber is being circulated through the dialyzer. With such an arrangement, it is possible to achieve a nearly continuous and uniform flow of dialysis fluid through the dialyzer. The flow is only momentarily interrupted during the switching over from one of the balance chambers to the other balance chamber. Such systems having two balance chambers, are, for example, described in the German Patent Publication 2,838,414, corresponding to U.S. Pat. No. 4,770,769 (Schael), issued Sep. 13, 1988.
In contrast to the continuously operating dialysis systems or balancing systems having two balance chambers, systems having a single, large balance chamber or fluid reservoir chamber also exist. Such a system is described, for example, in the German Patent Publication 2,544,258 or can be represented by the dialysis apparatus marketed under the Tradename and Model Monitral by the company Hospal AG located at Basel, Switzerland. In dialysis apparatus having such a large single balance chamber, the balance chamber is typically filled as quickly as possible in a first operating cycle. Then, in a second operating cycle, which is carried out over a substantially longer period of time, the dialysis fluid is circulated from the filled balance chamber through the dialysis fluid chamber of the dialyzer. In that manner, it is achieved that the time interval during which dialysis fluid does not flow through the dialyzer is kept relatively short.
Both of the above described systems are very costly and complicated, both in terms of the technology of the apparatus and instrumentation, as well as the technology of the control arrangements. On the one hand, the method and apparatus using a double balance chamber requires an extra balance chamber and an extra set of associated valves and conduits as compared to the single balance chamber method and apparatus. On the other hand, the method and apparatus using a single balance chamber makes high demands on the filling performance, which can lead to an unusually early wear and possible failure of the fluid pumps.